PONTIANAK (Kalimantan) • The sweltering backwaters of Indonesian Borneo have become the unlikely ground zero for the global production and export of kratom, a tree leaf hailed by some as a miracle cure for everything from opioid addiction to anxiety.

Part of the coffee family, the leaf has been used for centuries in South-east Asia for its pain-relieving and mildly stimulating effects.

But it is now sold in powdered form and exported worldwide – alarming some health regulators who have raised concerns about its safety.

Kratom stimulates the same brain receptors as morphine, although it produces much milder effects.

“I take kratom and have had no problems. Every strain has its benefits – some help you relax, others can treat insomnia or treat drug addiction. Some help increase stamina,” grower Faisal Perdana told Agence France-Presse.

Said fellow farmer Gusti Prabu, who now exports 10 tonnes of the drug a month: “Our ancestors used kratom and there were no negative side effects. It can help eliminate drug addiction and help people detox(ify).”

But its popularity is causing concern – the drug is unregulated, and has had little clinical testing to assess its safety or side effects.

Kratom is banned for consumption in Indonesia, Malaysia and Thailand, though Indonesia allows its export in an unprocessed form.

Health authorities in the United States – the drug’s top export market – have linked consumption of the plant and its derivatives to dozens of deaths, warning it could aggravate a deadly opioid epidemic gripping parts of the country.

Compounds found in kratom are opioids, which expose users to the same risks of addiction and death as illicit opiates, according to the United States Food and Drug Administration (FDA).

But for farmers in Kapuas Hulu, West Kalimantan – the centre of production – demand for kratom is such that they have moved away from traditional commodities, such as rubber and palm oil, to growing the tree as a major cash crop.

At the main post office in Pontianak, the key trading post for this part of Indonesian Borneo, it is clear the health warnings have done little to dampen interest.

“Around 90 per cent of our shipments from West Kalimantan province are Kratom that’s been sold to the United States,” post office head Zaenal Hamid said.

As many as five million Americans use the drug and that number is growing, according to the American Kratom Association.

Data from 2016 showed that the region was shipping some 400 tonnes abroad every month – worth about US$130 million (S$176 million) annually at current global prices of US$30 a kilogram.

Most kratom customers are reached through online platforms such as Facebook, Instagram and Chinese e-marketplace Alibaba.

The trend for alternative medicine has been credited with increasing interest in kratom from Europe and America, where it is usually consumed as a tea or in capsules.

As the US struggles with an opioid addiction, fuelled by prescription painkillers and street drugs like heroin and fentanyl, kratom is legal in 43 states.

But the FDA is pushing for greater restrictions and has put an import alert on it, which means shipments entering the US can be confiscated.

It warned consumers not to use the drug and said it was “concerned that kratom appears to have properties that expose users to the risks of addiction, abuse, and dependence”.